The present invention relates to the art of data communications and specifically to the demodulation of serial-binary data signals from a phase modulated carrier.
The transmission of digital data commonly takes place over telephone communication facilities. Voice-grade telephone channels are convenient for such purposes; however, telephone channels are not the perfect data transmission media users would otherwise desire. Frequency limitations, nonlinear phase distortion, and nonlinear amplitude distortion are common problems in using telephone channels. Data sets currently in use employ multiphase modulation, either four-phase or eight-phase with some form of line equalization, to overcome many of these telephone channel deficiencies.
There are two conventional methods used to demodulate digital multiphase signals: coherent detection and differential detection. In coherent detection (also called fixed reference detection), the absolute carrier phase reference must be recovered at the receiver so that the carrier phase may be compared or subtracted from the modulated signal phase. Differential or comparison detection is usually used in data sets which operate over telephone lines, since telephone lines are rarely stable enough to preserve the absolute carrier phase. In differential systems, the digital data is encoded in terms of phase changes, and detection is performed by comparing the phase of successive signal samples.